1. Field of Invention
Embodiments of the invention relate generally to memory, and more particularly, to STT-MRAM cells configured for symmetrical and unidirectional programming
2. Description of Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light and not as admissions of prior art.
Magnetic Random Access Memory (MRAM) is a non-volatile memory technology based on magnetoresistance. Unlike typical Random Access Memory (RAM) technologies which store data as electric charge, MRAM data is stored by magnetoresistive elements. Generally, the magnetoresistive elements in an MRAM cell are made from two magnetic regions, each of which holds a magnetization. The magnetization of one region (the “pinned region”) is fixed in its magnetic orientation, and the magnetization of the other region (the “free region”) can be changed by an external magnetic field generated by a programming current. Thus, the magnetic field of the programming current can cause the magnetic orientations of the two magnetic regions to be either parallel, giving a lower electrical resistance across the magnetoresistive elements (“0” state), or antiparallel, giving a higher electrical resistance across the magnetoresistive elements (“1” state) of the MRAM cell. The switching of the magnetic orientation of the free region and the resulting high or low resistance states across the magnetoresistive elements provide for the write and read operations of the typical MRAM cell.
A spin torque transfer MRAM (STT-MRAM) cell is another type of memory cell which is programmed by changing the magnetization of magnetoresistive elements. The STT-MRAM cell is written by transmitting a programming current through a magnetic cell stack including a free region and a pinned region. The programming current is polarized by the pinned region. The spin-polarized current then exerts the torque on the free region, switching the magnetization of the free region. The magnetization of the free region can be aligned to be either parallel or antiparallel to the pinned region, and the resistance state across the stack is changed.
Typically, a bidirectional programming current is used to change the free region magnetization from parallel to antiparallel and from antiparallel to parallel with the magnetization of the fixed region. To change the free region magnetization from parallel to antiparallel to that of the fixed region, electrons flow from the free region to the fixed region, and reflected electrons from the fixed region having opposite spins to the magnetization of the fixed region are used to switch the magnetization of the free region. Less electrons having an opposite spin to the magnetization of the fixed region may be reflected from the fixed region compared to electrons which are spin-polarized as they flow through the fixed region. Therefore, changing the free region magnetization from parallel to antiparallel with the magnetization of the fixed region generally uses a greater programming current (e.g., to have a greater number of reflected electrons having an opposite spin to the magnetization of the fixed region) than changing the free region magnetization from antiparallel to parallel with the magnetization of the fixed region. Such asymmetry in cell programming may result in various inefficiencies in manufacturing and/or operating an STT-MRAM cell.